Monochromators have already been well known wherein the dispersion member was a reflection grating that forms the image of the incident beam of radiation passing through an entrance slit onto an exit slit. The wavelength of the emitting beam of radiation depends on the position of the entrance slit, the exit slit and the grating, and can be described by the following generalized grating-equation: ##EQU1## where .alpha. is the angle included by the beam of radiation incident to the grating having passed through the entrance slit and the normal to the grating, .beta. is the angle included by the beam of wavelength .lambda. reflected by the grating toward the exit slit and the normal to the grating, a is the spacing between the rulings of the grating, and K is a positive integer referring to the order of the diffraction.
In the known spectrophotometers two of the three basic elements of the monochromator (entrance slit, grating, exit slit) are stationary and the third one is moved to change the wavelength. In the case, when one of the angles .alpha. and .beta. is constant and the other one is changed, or with a Littrow-arrangement of the entrance and exit slits, both angles are changed simultaneously so that .alpha.=.beta., and a sinusoidal relation exists between the variable angle and the wavelength. To attain a linear scale--which is being especially desirable in the ultraviolet, the visible and the near infrared regions--the sinusoidal relation should be linearized.
In case of some known spectrophotometers a Scotch-yoke mechanism is utilized for the purpose of this linearization, wherein the sine of the rotational angle of the crankshaft is linearly proportional to the displacement of the yoke. In case of some other spectrophotometers a profiled disc is mounted on the shaft of the rotating member resulting in a translation or rotation linearly proportional to the sine of the angle of rotation. With these solutions of the linearization problem technological difficulties may arise in designing and reproducing the special profiles, furthermore the clearances and the distortions caused by uneven wear may result in errors.
Recently--especially in the low energy level ultra-violet and near infra-red regions--the application of the concave reflection gratings has become widely spread, that form the image of the entrance slit on the exit slit, and provide a high relative opening (diameter divided by the focal distance) as a result of the special ruling, performing also the function of the collimator. In these monochromators both the entrance and the exit slit might be positioned along the so-called Rowland-circle so that the image formation will be sufficiently sharp. As is well known, the diameter of the Rowland-circle is the distance between the center of curvature of the concave reflection grating and the center-point of the concave grating (the intersection of the optical axis and the grating, so-called vertex). So in this case linearizing the sinusoidal relationship for example by rotating the shaft of the concave grating by means of a Scotch-yoke mechanism is not yet sufficient, but the position and even the direction of one of the slits should also be varied in the course of the rotation. In case of the known concave reflection grating spectrophotometers the exit slit does not coincide with the Rowland-circle within a significant part of the spectral range. Consequently the spectral resolution is not satisfactory at these wavelengths on the one hand, and on the other hand a linear wavelength scale cannot be provided merely by linearizing the sinusoidal relationship.